This application involves an improvement to the design of hydraulically motivated surgical tables. The invention is particularly applicable to surgical tables used in hospital operating rooms, although it will be appreciated that the invention has broader applicability to any task demanding precision positional control of a live subject or inanimate workpiece. Other possible applications include, without limitation, dentists' chairs, and veterinary equipment.
Surgeons have, in recent years, employed operating tables capable not only of vertical adjustment, but also tilting the plane of the operating table to various degrees of pitch or yaw about the central axes. Hydraulic control systems have been employed to achieve a continuous adjustment of a patient's body position to accommodate the ergonomic needs of the surgeon, and the demands of the surgical procedure employed.
The basic design of such tables features a double-acting hydraulic cylinder that extends and retracts to adjust the position of the table surface. The cylinder moves to its desired position by means of pilot operated check valves that deliver hydraulic fluid to either side of the cylinder. When the cylinder arrives at its desired position, this position is maintained by back pressure against the check valves.
When the check valves leak, however, one of two undesirable things can occur. First, the hydraulic pressure could be lowered on the side of the cylinder with the leaky valve, causing the table to shift under its load. Second, if the load were balanced, the fluid could leak out of one side, leaving only low pressure, compressible air. Then, when the balance of forces shifted, the table could suddenly fall in one direction or the other.
An early attempt at a solution to the foregoing problem was the addition of an auxiliary mechanical locking device. When the locking device was manually released, however, a sudden movement could result from a pre-existing hydraulic pressure imbalance. Thus, it is desirable to have mechanical locking capability to maintain a selected position despite leaky check valves, but which provides smooth movements when the hydraulic system is next activated.